Venus Express – 24th May 2007

This evening’s meeting started with our Chairman Jim Mehta giving us some of the latest astronomical news, Chris Suddell showing us a long-exposure image he’d captured last night from the field at Tuesnoad, and Tony Sizer explaining the complexities of the new planetarium at Greenwich.

The main talk about Venus Express was given by Dr. Andrew Coates from UCL and MSSL. He began by describing some obvious differences between our Solar System’s four inner planets. Mercury and Earth have magnetic fields strong enough to protect them from the Solar Wind. Venus and Mars do not have significant magnetic fields of their own any more, so they feel the full force of the Solar Wind. Venus also has a very dense super-rotating atmosphere. The planet itself rotates very slowly in the “wrong” direction, but its atmosphere is rotating much faster, so that it has vortices at its poles. Parts of its atmosphere go all the way round the planet in as little as four Earth days.

The Venus Express mission was designed to be able to look in depth at Venus’ atmosphere, from its lowest levels to the ionosphere and its interaction with the Solar Wind. Unfortunately one of its seven main instruments is not working, but the remaining six are producing lost of information. The ASPERA instrument is used to determine the charge and composition of the plasma the Venus Express craft is passing through. The elliptical orbit maintains a fixed orientation with respect to the stars, so as Venus goes round the Sun the craft can build up a picture from a full range of orientations. It can directly measure the planet’s bow-shock and related phenomena, and detect the quantity and composition of the plasma being stripped from Venus’ atmosphere by the Solar Wind. It seems Venus is having much more matter stripped from it than previously expected. Curiously, a similar instrument on Mars Express has determined that Mars is losing less matter than expected. Similar instruments are also in place on Cassini to measure the effects on Titan’s atmosphere of Saturn’s magnetosphere, and on Rosetta, which will go into orbit around a comet.

Other instruments on Venus Express have taken detailed images of the planet’s atmosphere at many levels, and the complex double-vortex at the planet’s south pole is particularly fascinating. Future highlights include a fly-by of Messanger on its way to Mercury. Briefly, Venus will be being studied by two spacecraft at the same time. There are also opportunities for amateur astronomers to help the mission by imaging Venus using particular filters at set times. For more information, see the ESA website at sci.esa.int.

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